Namibian and South African Horse resource September 2013 update (Melanie Smith and Dave Japp )

Historical Perspectives on Horse Mackerel in the Benguela Ecosystem

In this year’s report for Namibia we focus on the environmental effects on horse mackerel and the historical catches in the region.

Understanding the historical development of the horse mackerel fishery in Namibian waters gives an interesting perspective. Since the mid 1970’ s Cape horse mackerel ( capensis) has been the largest contributor to catches made within the Exclusive Economic Zone (EEZ) of Namibia, exceeding 600 000 tonnes per annum in the 1980's. It is believed that the increase in abundance of this species was associated with the collapse of the pilchard (Sardinops. sagax) stock in Namibian waters towards the end of the 1960’s and also the decline of the hake (Merluccius capensis) stock over the past two decades.

The commercial pelagic fishery off Namibia (within 12 nautical miles from the coast) dates back to the late 1940’s. The pelagic industry was initially based on pilchard using a net with a mesh size of 28mm but since the collapse of the pilchard fishery the pelagic Cape horse mackerel fishery saw significant growth during the 1970's and 1980's. The current directed midwater trawl fishery, outside the 12 nautical mile zone, commenced in 1961 and was dominated initially by vessels from the former Soviet Union. However, interest by international trawler fleets in the rich fish resources off Namibia, targeting of horse mackerel increased in the early 1970’s. Countries from the former East Block that joined the already existing fishing operation of the Soviet Union off Namibia included Poland, Romania, Bulgaria, East Germany, as well as vessels from as far as North Korea, Japan, and Cuba. These midwater trawlers were operating with a smaller cod end mesh (60mm) than is currently permitted in Namibia and South Africa (75mm).

Environmental Influences on the Horse mackerel fisheries in the region

Horse mackerel are commonly found throughout the world’s oceans. They are a pelagic species (found in midwater from the surface of the sea to the sea floor). Historical research conducted by numerous taxonomists has confirmed that two distinct species of horse mackerel occur in Namibian and Angolan waters namely the Cape horse mackerel (Trachurus capensis, Castelnau, 1861), and the (Trachurus trecae, Cadenat, 1949). Both species are common in northern Namibia and southern Angolan waters and their distribution is dependent on the position where the cold (Benguela Current) and warm (Angola Current) water masses meet. The distribution of Cape horse mackerel (T. capensis), which favours cold water masses less than 20°C, extends southwards from the Cunene River as far as the east coast of South Africa. Cunene horse mackerel (T. trecae) which favours warmer water above 20°C, occurs northwards from the Cunene River to the equatorial regions.

The distribution of these two species in the Benguela Ecosystem is therefore influenced by environmental variability (Figure 1). Much research on environmental effects on fisheries in the Bengulea Ecosystem has and is being carried out. As with other small pelagic species (sardine, anchovy etc.), the behaviour and availability of horse mackerel affects the commercial horse mackerel fisheries more so than other sectors (such as hake).

Cunene River Cunene River

Lüderitz Lüderitz

Orange River Orange River

Figure 1. Monthly Sea Surface Temperature (SST) in the northern Benguela region. NOAA satellite images taken in February (left image) and August (right image). (after Klingelhoeffer, 2005 – Population dynamics and development of a sustainable fishery for the cape horse mackerel Trachurus capensis, in the northern Benguela upwelling system).

A Sea Surface Temperature (SST) satellite image (NOAA 14) of the monthly mean in February 2000 showing warm surface temperatures over much of the northern and central Benguela Current upwelling system indicating suppressed upwelling, and late August 1999 showing strong upwelling along the entire Namibian shelf (Figure 1). Orange to red indicates surface water ranging between 20° C to 23° C; light blue to dark blue indicates upwelled water ranging between 12° C - 15° C. This variability in turn also affects the migratory patterns of horse mackerel, in particular recruitment to the commercial fisheries. The likely migration route and nursery area of Cape horse mackerel is shown in the Figure 2. The environmental variability can therefore profoundly affect not only the potential for successful recruitment to the commercial fisheries, but also the movements of the stocks (Figure 2).

Figure 2. The migration routes and the nursery areas for horse mackerel taken from the synthesis of the scientific input into the Angola Benguela Currents Front Workshop – April 2006.

The Angolan Connection

There has been considerable interest in the commercial availability of horse mackerel in northern Namibia and southern Angola. In recent years regular “transboundary” surveys between these two countries has been conducted. The management of transboundary species such as horse mackerel and hake is the mandate of Benguela Current Commission (BCC) - which is in process of being ratified by Angola, Namibia and South Africa. Again, the distribution of the two species varies with the location of the Angola Benguela Front Zone (ABFZ). The ABFZ migrates southwards annually and introduces warm, saline and nutrient-poor Angolan water into Namibian coastal waters to a depth of about 100 m, and is associated with a relaxation of the equator ward, upwelling-favourable wind- stress. This phenomenon sometimes leads to mass mortalities of mostly inshore marine organisms including linefish species, squids and mussels.

As the Cunene horse mackerel occurs mostly north of the front, with the distribution extending to northern Angolan waters, it is rarely found in the Namibian waters. The Cape horse mackerel stock is generally distributed from Tombwa in southern Angola (16S) and throughout Namibian waters, but abundance is generally high (95%) in the Namibian waters, and even higher during the summer months when the Angola-Benguela front shifts southwards. Further, the horse mackerel stocks were also heavily overexploited in Angolan waters prior to independence, and like Namibia, Angola has initiated a rebuilding process of their horse mackerel, including the banning of the large industrial midwater trawlers similar to those used in Namibia.

In Angola the fisheries for horse mackerel are much more diverse than their southern neighbours. For 2013, out of the allowable catch of 65 000 t, some 15000 t is allocated to Cunene horse mackerel and the balance to Cape horse mackerel. In Namibia a TAC of 320 000 t has been declared for 2013 with only a small amount accommodated for the pelagic purse seine sector

The South African Horse Mackerel Fishery

Adult Cape horse mackerel is targeted by the midwater trawl fishery on the South Coast (offshore east of Agulhas Bank) at the extreme eastern range of the species (Figure 3). Only one dedicated midwater vessel, the MFV Dessert Diamond operates in the South African waters and effort is limited to a narrow area on the South Coast (Figure 3).

Figure 3. Distribution of fishing effort and fisheries targeting horse mackerel in South African waters.

Horse mackerel is also caught as bycatch in the small pelagic purse seine fishery (targeting anchovy and sardine) which operate inshore on the West Coast where a high abundance of juvenile horse mackerel (>10cm) is incidentally caught (Figure 3). Similarly, the distribution of demeral trawl activity occurs in areas where adult horse mackerel (>30cm) Figure 4. Annual horse mackerel catch landed by the pelagic fishery are incidentally caught (Figure 2000-2013 (till July 2013). The average catch and annual PUCL are 3). It is believed that the West also shown (Singh, et al. 2013)

Coast juveniles mainly recruit to the stock off the South Coast and it is for this reason that purse- seine effort (from the small pelagic sector) is strictly controlled to sustain a strong adult population on the South Coast through Precautionary Upper Catch Limits (PUCL). The current PUCL for horse mackerel adults is set at 44 000t, of which 31 500 is allocated to the midwater trawl fishery while the remaining 12 500t forms the demersal trawl bycatch reserve. The PUCL for the horse mackerel juveniles in the small pelagic fishery has been set at 13 000t, the highest level since 2000 (catch series shown in Figure 4). This is a result of unusually large numbers of juvenile horse mackerel occurring on the West Coast and to prevent the purse-seine fishery from being closed before fulfilling their anchovy quota the sector requested an increase in the PUCL to enable continued fishing of anchovy (Figure 4).

In 2011 a comprehensive horse mackerel assessment incorporating catch data (Figure 5) and length frequency data from both the commercial and fishery research surveys was completed. The results indicated that there was a 20% increase in horse mackerel abundance over the last five years which scientists believe was primarily due to good recruitment and management in recent years. Figure 5. Updated demersal and midwater catches from 2000-2013 separated by coast. The catch for 2013 is not complete. (Singh, et al. 2013)